Quick diagnosis
What this code is telling you
Most likely trigger
Refrigerant leak in outdoor coil or copper line set
Recommended replacement
Low Pressure Cutout Switch
Diagnostic path
4 ordered checks
Part fit
Exact model number required
Safety first
Before opening the appliance
Unplug the appliance before removing panels or testing internal components. Do not bypass safety switches or live-test high-voltage, gas, refrigerant, or combustion components unless you are qualified to do so.
Read the full safety disclaimerRoot cause
Why the control reported this error
The heat-pump control reports this fault when a monitored pressure, temperature, communication, or compressor condition is abnormal. Begin with refrigerant leak in outdoor coil or copper line set; also verify extremely dirty indoor air filter causing evaporator freeze-up and defective low pressure switch sensor before replacing low Pressure Cutout Switch.
Symptoms
What you may notice
- ✓Outdoor compressor shuts down completely
- ✓System switches to auxiliary backup heat
- ✓Thermostat alerts E121 Low Pressure Fault
Possible causes
Check the most likely causes first
- 1
Refrigerant leak in outdoor coil or copper line set
Start here when it matches your symptoms.
- 2
Extremely dirty indoor air filter causing evaporator freeze-up
- 3
Defective low pressure switch sensor
- 4
Thermal expansion valve (TXV) stuck closed
Diagnostic sequence
Work through these checks in order
Stop when you find the fault. Do not perform a step that requires skills or live testing you are not qualified to do.
- 1
Check 1
Turn off cooling/heating and check air filter for dirt saturation.
- 2
Check 2
Inspect indoor coil for frost or ice buildup; melt completely before restarting.
- 3
Check 3
Contact certified HVAC technician to check standing system pressures and test for leaks.
- 4
Check 4
Replace defective low pressure cut-out switch.
Replacement part reference
Low Pressure Cutout Switch
Exact OEM part number
Model number required
Fitment note
The exact OEM part number varies by model and production revision. Verify the complete model number before ordering.
Verify the appliance model before ordering.
Manufacturers can use different parts under the same error code across model families and production revisions.
Where do I find my model number? →FixCodeDB may earn a commission from qualifying purchases made through affiliate links.
Common questions
Quick answers for this fault
What should I check first for E121?
Start with refrigerant leak in outdoor coil or copper line set. If that checks out, continue through the diagnostic sequence above rather than replacing a part based on the code alone.
Does E121 always mean Low Pressure Cutout Switch is bad?
No. An error code identifies the condition the control detected, not a guaranteed failed part. Confirm the likely causes and wiring first. The exact OEM part number varies by model and production revision. Verify the complete model number before ordering.
When should I stop troubleshooting E121 myself?
Stop if the next step requires live electrical testing, sealed-system work, gas or combustion service, or any procedure outside your training. Unplug the appliance before removing panels or testing internal components. Do not bypass safety switches or live-test high-voltage, gas, refrigerant, or combustion components unless you are qualified to do so.
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